About the Pulley Speed Calculator
This pulley speed calculator solves the basic belt-drive relationship D₁ × N₁ = D₂ × N₂. Enter the driver (motor) pulley diameter and speed with the driven pulley diameter to get the driven RPM, or enter the speed you want and get the pulley size that delivers it. A third mode finds the driver pulley size for a given driven pulley. Every result also shows the speed ratio and the belt speed in feet per minute and metres per second.
It is handy for setting up a drill press, lathe, bandsaw, fan, blower, compressor, go-kart or any machine driven by V-belts, timing belts or flat belts. Diameters can be entered in inches or millimetres — the ratio does not depend on the unit, as long as both pulleys use the same one.
The calculation assumes no belt slip. Real V-belt drives typically run 1–3% slower than the ideal driven speed. For V-belts, use the pitch diameter (roughly the outside diameter minus the belt’s height) for the most accurate result.
How to use the pulley speed calculator
- 1Choose what to solve for: driven RPM, driven pulley size or driver pulley size.
- 2Pick inches or millimetres for diameters.
- 3Enter the motor (driver) speed from the nameplate.
- 4Enter the known pulley diameters and, if needed, the target driven RPM.
- 5Read the result, the speed ratio and the belt speed.
Formula and method
With no slip, every point on the belt moves at the same linear speed, and that speed equals the rim speed of each pulley: π × diameter × RPM. Setting the two rim speeds equal gives D₁N₁ = D₂N₂, so pulley speed is inversely proportional to diameter. A driven pulley twice the size of the driver turns at half the speed.
The speed ratio is D₂ ÷ D₁ (equal to N₁ ÷ N₂). Belt speed is π × D₁ × N₁, converted to feet per minute (diameter in feet) or metres per second (diameter in metres ÷ 60). Torque scales the opposite way to speed, so a 2:1 reduction roughly doubles the torque available at the driven shaft.
- D₁, N₁
- Driver pulley diameter and speed (RPM)
- D₂, N₂
- Driven pulley diameter and speed (RPM)
Worked examples
1,750 RPM motor, 4" driver, 8" driven pulley
Driven speed = 4 × 1,750 ÷ 8 = 875 RPM, a 2:1 reduction. The belt travels π × (4/12) ft × 1,750 = 1,833 ft/min, or about 9.3 m/s.
Pulley size for 600 RPM from a 3" pulley on a 1,725 RPM motor
Driven diameter = 3 × 1,725 ÷ 600 = 8.625 inches, so an 8.5- or 9-inch stock pulley gets you close (609 or 575 RPM). The ratio is 2.875:1 and the belt runs at about 1,355 ft/min.
Metric: driver size for 900 RPM with a 250 mm driven pulley on a 1,450 RPM motor
Driver diameter = 250 × 900 ÷ 1,450 = 155.2 mm, a 1.61:1 reduction. The belt speed is π × 0.1552 m × 1,450 ÷ 60 ≈ 11.8 m/s.
Frequently asked questions
How do you calculate pulley RPM?+
Multiply the driver pulley diameter by its RPM and divide by the driven pulley diameter. A 4-inch pulley on a 1,750 RPM motor driving an 8-inch pulley gives 4 × 1,750 ÷ 8 = 875 RPM at the driven shaft.
Does a bigger pulley make it faster or slower?+
A bigger driven pulley makes the driven shaft turn slower (with more torque). A bigger driver pulley on the motor makes the driven shaft turn faster. Speed is inversely proportional to the pulley diameter it is attached to.
Should I use outside or pitch diameter?+
For V-belts, use the pitch diameter — where the belt’s load-carrying cords ride — which is smaller than the outside diameter by roughly the belt height. For quick estimates with similar-size pulleys, outside diameters give a ratio that is close enough.
How much do belts slip?+
A properly tensioned V-belt typically runs 1–3% slower than the theoretical speed due to creep and slip. Timing (synchronous) belts and chains do not slip, so their speed ratio is exactly the tooth-count ratio.
What is a safe belt speed?+
Classical V-belts work best around 4,000–5,000 ft/min, and standard cast-iron sheaves are usually limited to about 6,500 ft/min (33 m/s) because of rim stress. Check the belt and sheave manufacturer’s limits for your drive.